Hydration needs
Enter the conditions, your effort and your weight: we give you millilitres per hour and a sodium target. If you weighed yourself around a run, the section below gives the exact number.
For example 3:30 (3 hours 30 minutes) or 45:00 (45 minutes).
Humidity
Effort
Fluid per hour
680-890 ml
Estimated sweat loss 1.05 L/h · 300-750 mg sodium per hour · carry 0.7-0.9 L in total
I weighed myself (more accurate)
For example 90
These are general ranges, not medical advice. Test any new fuelling plan in training, never for the first time in a race.
The estimate starts from a low base sweat rate in cool weather and rises with every degree above 10 °C, then scales with body weight, humidity and effort. The recommendation covers 65-85% of the estimated loss, because clearing the deficit while running is neither realistic nor necessary, and it is capped at one litre per hour because of absorption limits. The sodium range assumes 300-700 mg of sodium per litre of sweat. If you weighed yourself the maths is simpler: sweat loss = (weight before - weight after) + fluid drunk.
These are general ranges and assume normal conditions. Trail, heat, elevation gain, wind and technical ground all make the real time longer.
Frequently asked questions
(11)
How much fluid loss is a problem?
Losing more than 2% of body weight hurts performance and becomes risky in heat. For a 70 kg runner that is about 1.4 kg.
Should I drink as much as I lose?
No, and trying usually causes stomach trouble. Replacing 70-80% of the loss is enough for most runners; let thirst guide the rest.
How much do runners sweat per hour?
The typical range is 400-1,200 millilitres per hour, and in hot, humid conditions measurements above 2 litres occur. The range is so wide that general advice is useless: your own weigh-in is the only reliable number.
When do I need electrolytes or salt?
In races over an hour and in hot weather. Sweat sodium varies from 200 to 1,500 milligrams per litre between individuals; if you finish with salt marks on your kit you are at the high end, and drinking plain water alone dilutes your blood sodium further.
How much should I drink before a race?
Aim for 5-7 millilitres per kilogram of body weight 2-4 hours before the start, which is 350-500 millilitres for a 70 kg runner. Pale urine means you are ready; loading up in the last half hour only buys you a toilet stop.
Can I plan hydration without weighing myself?
Yes, that is what the estimate on this page is for: it predicts hourly sweat loss from temperature, humidity, effort and body weight. For a 70 kg runner at 20 °C and steady effort it lands around one litre per hour, and above 1.5 litres in hot, humid conditions. Individual sweat rates vary up to threefold, so treat it as a starting plan and calibrate it with a weigh-in when you can.
How much sodium should I take per hour?
The common range is 300-700 mg per hour, and you move towards the top of it as you sweat more, which is why this page scales sodium with your sweat rate: a litre of sweat typically carries 300-700 mg of sodium. In races over an hour in the heat, drinking water alone dilutes your blood sodium (hyponatraemia); salt capsules, an electrolyte drink or salty food all do the same job.
Should I drink more than a litre per hour?
Usually not. Gut absorption tops out around 1-1.2 litres per hour for most runners, and anything beyond that sloshes and causes nausea. If your sweat rate is higher, the goal is not to clear the deficit but to stay under 2% body weight loss and replace the rest after the finish. That is why the hourly recommendation here is capped at one litre.
What is hyponatraemia and what are the signs?
It is dilution of the sodium in your blood, usually from drinking more plain water than you lose over a long race. Signs are bloating, nausea, headache, confusion and in severe cases seizures; finishing heavier than you started is the clearest warning. If in doubt, stop drinking water, take something salty and seek medical help.
Is urine colour a reliable check?
It is rough but useful: pale straw is good, dark yellow suggests dehydration and completely clear suggests you are drinking too much. B vitamin supplements darken it misleadingly. The accurate measure is weighing yourself before and after a run.
Do coffee and alcohol ruin hydration?
Habitual coffee does not meaningfully change your daily fluid balance; its diuretic effect is largely offset by the fluid it contains. Alcohol is different: a drink the night before a race costs both fluid and sleep, and you feel it in sweat loss the next day.